1. Field of the Invention
The present invention relates to telephony, and more particularly relates to telephone line surge protector modules.
2. Description of the Prior Art
Telephone line surge protector modules are well-known devices. One such device is described in U.S. Pat. No. 3,947,730 to DeLuca et al., issued Mar. 30, 1976, the disclosure of which is incorporated herein by reference. Protector modules normally accommodate both the telephone line tip (conversation) and ring circuits.
A typical protector module includes a transient over-voltage device designed to short the affected circuit to ground in the event of an over-voltage condition. Early modules employed a simple air gap with a pair of arcing electrodes; dielectric breakdown of the air in the gap allowed arcing to ground during the over-voltage condition. Later devices have employed sealed tubes incorporating a pair of spaced-apart arcing electrodes and containing a gas of known dielectric strength.
Recent developments in telephone switch gear involve the use of electronic digital switches. Although such switches offer many advantages over prior art devices, they suffer from greater sensitivity to high voltage spikes and current surges. Accordingly, there is much interest in developing a telephone line surge protector module that incorporates a transient over-voltage device which can actuate more rapidly than prior-art gas-tube devices. Such modules have been developed, using fast-acting solid state over-voltage protection devices. These solid state devices, although faster than gas tubes, are unable to tolerate as much energy dissipation as gas tubes.
In addition to over-voltage protection, telephone line surge protector modules typically include protection against excessive current, as well, most often by a thermally activated device responsive to Ohmic heating produced by the over-current. One well-known thermal device is the heat coil, wherein a coil of resistive heating wire wound about a bobbin carries the circuit current. An elastically-biased ground contact is fastened to the bobbin with a low melting point solder. When the circuit current exceeds a predetermined value, corresponding to an over-current condition, the Ohmic heat generated in the coil melts the low melting point solder and the elastically-biased ground contact is free to short the affected circuit to ground. In some applications, it is desirable to open the circuit under spike and surge conditions, rather than grounding it.
In view of the increased delicacy of electronic digital switches used in telephone switch gear, as well as the low energy dissipation tolerance of solid-state over-voltage protection devices frequently employed with such switches, there is a need for a fast-acting, high resistance heat coil assembly to provide over-current protection in telephone line surge protector modules. Prior art heat coils suffer from the disadvantage of limited electrical resistance due to thermal and structural constraints, thereby limiting the Ohmic heating (and lengthening actuation time) for a given current level.